There is considerable interest in using rare earth doped fiber amplifiers to amplify weak optical signals for both local and trunk optical telecommunications networks. The rare earth doped optical amplifying fibers are found to have low cost, exhibit low-noise, relatively large bandwidth which is not polarization dependent, substantially reduced crosstalk problems, and low insertion losses at the relevant operating wavelengths which are used in optical communications. Contemplated rare earth doped optical fiber amplifiers can be coupled end-to-end to a transmission fiber, and transversely coupled, through a directional coupler, to a laser diode pump. The directional coupler is designed to have a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength so that maximum pump energy is coupled to the amplifier with minimal signal loss. When the amplifying medium is excited with the pump laser, signal light traversing the amplifier experiences gain. The pump energy may be made to propagate either co-directionally or contra-directionally relative to the signal energy, depending upon whether any remaining unconverted pump light can be more conveniently filtered at the transmitter or the receiver.
A complicating factor in the design of rare earth doped optical amplifiers involves the difference between the various parameters necessary to optimize the performance of the amplifier and those necessary to optimize the performance of the associated transmission fibers. These differences, which arise from the different functions performed by the optical amplifier and the transmission fiber, result in significant signal loss as the signal is transmitted from the transmission fiber to the amplifying fiber, and therefore place a premium on the efficiency of the amplifying fiber which restores the signal to its previous levels. In the transmission fiber, waveguide dispersion must be minimized in order to maximize bandwidth and minimize loss, thereby maximizing the spacing between repeaters. However, in the amplifying fiber, as opposed to the transmission fiber, the major concern involves high gain, high saturation power, and low noise, all with minimal pump powers. Exemplary signal losses that can occur because of the different optimal parameters for the transmission and amplifying fibers are splicing losses due to mode mismatch because the signal mode size may be significantly different for these two fibers.
To date, erbium fiber amplifiers appear to have the greatest potential for the high amplification necessary to overcome the signal losses due not only to normal signal processing but due to the mismatch between the transmission and amplification fibers mentioned above. Erbium doped fiber amplifiers operate at .lambda.=1.53 .mu.m which is of particular interest for optical communication systems because, in this wavelength region, the amplifiers exhibit low insertion loss, broad gain bandwidth (approximately 30 nm) and polarization insensitive gain. Such amplifiers, pumped at .lambda.=1.48 .mu.m can have a gain as high as 26 dB but require as much as 76 mW of launched pump power. A higher gain together with a lower value of pump power is preferred.